Carboxylic acids and alcohols are both homologous series found in organic chemistry. Describe two differences and one similarity between carboxylic acids and alcohols.

AQA A-Level Chemistry (7405) — 3.3.8 Aldehydes and ketones (A-Level only) · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

1. Carboxylic acids contain the –COOH functional group, whereas alcohols contain the –OH group.
2. The first member of the carboxylic acid series is methanoic acid (formic acid) and the first member of the alcohol series is methanol.
3. Both series are homologous, so each successive member differs by a –CH2– unit and they show similar trends in physical properties.

Examiner tips

  • Use the exact functional group symbols –COOH and –OH. Mention the first members (methanoic acid and methanol) to secure the difference marks. State that both are homologous series to gain the similarity mark.

Common mistakes

  • Writing ‘ethanoic acid’ instead of ‘methanoic acid’ for the first member. Confusing the functional groups or omitting the –OH group for alcohols. Failing to mention that both series are homologous, losing the similarity mark.

Mark scheme (5 marks)

  1. Difference 1: Carboxylic acids have the functional group –COOH whereas alcohols have the functional group –OH
  2. Difference 2: The first member of carboxylic acids is methanoic acid whereas the first member of alcohols is methanol (accept any correct contrasting named members, e.g. ethanoic acid vs ethanol)
  3. Similarity: Both carboxylic acids and alcohols belong to a homologous series / both have similar chemical properties within their series
  4. Both carboxylic acids and alcohols contain carbon, hydrogen and oxygen atoms in their molecules

Key terms in this question

homologous series · carboxylic acids · alcohols

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